(1) Field of the Invention
The present invention relates to a low-hygroscopic thermosetting sulfur-containing resin and resin plate, a shape-recoverable molded article, sulfur-containing coating material for forming low-hygroscopic coating film, and a sulfur-containing urethane adhesive having a long pot life and high water resistance.
(2) Description of the Prior Art
Urethane resins have excellent elasticity as well as high wear resistance, tensile strength and tear resistance, and are resistant to oxygen and ozone. In recent years, therefore, these resins have been widely used in the form of elastomers as materials for bumpers and mechanical parts.
Furthermore, the urethane resins are substantially transparent but are more hygroscopic than polymethacrylate resins and polycarbonate resins. Therefore, they are liable to warp and crack, and for this reason, they cannot be subjected to a hard coating agent treatment which is essential for the manufacture of windowpanes for vehicles, covers for lighting units, signboards, photodiscs, photocards and the like. Consequently, the urethane resins are not applied to these uses at present.
In addition, urethane coating materials have a beautiful appearance as well as high adhesion and wear resistance, and they are resistant to oxygen and ozone. For this reason, the urethane coating materials are utilized for high-quality woodwork as well as an outer decoration for buildings, airplanes and railway cars. Recently, it has been attempted to use them for coating automobiles.
The urethane coating materials also exhibit rapid curing and high luster, but their pot life is short and the hygroscopicity of a coating film made therefrom is greater than polyacrylate coating materials. Thus, urethane coating materials having low hygroscopicity in addition to weathering resistance are desired.
Urethane adhesives are used as various reactive-type adhesives. That is, the urethane adhesive contains a compound having an isocyanate group, which is active as to react with, e.g., a hydroxyl group contained or adsorbed in most substances, with the result that a very strong adhesive force is exerted. In addition, the urethane adhesives are generally excellent in chemical resistance, cold resistance and impact resistance, and therefore they are now used as adhesives for structures which require semipermanent bonding force and high-strength holding force.
As the urethane adhesives, the so-called two-part adhesives are widely utilized in which a three-dimensional cross-linked urethane is formed by the reaction of a polyisocyanate, a polyester polyol and then cured and a polyether polyol, obtain strong adhesive force. Heretofore, in order to improve the water resistance of bonding layers, a method using a specific kind of polyol and another method in which an epoxy resin having relatively good water resistance is simutaneously used have been proposed (Japanese Patent Publication No. 41-22758).
Since the urethane adhesives have high reactivity, their pot life in a liquid state is short and their workability is poor. Furthermore, the urethane resins have higher water absorption as compared with any other resin and a urethane linkage is substantially liable to hydrolyze, and therefore there is the problem that the urethane linkage in the bonding layers can be cleaved.
In a urethane resin bonding layer prepared by the above-mentioned conventional method in which water resistance is improved, 1.0 to 2.0% by weight of water is contained, even in the case of an example where the water absorption is lowest. In summary, the urethane adhesives still have the drawback that their adhesive force deteriorates when exposed to the repetition of dry and wet periods and when wetted directly with rainwater.